While Secretary of State Rex Tillerson fends off rumors he could be out of a job within weeks, he is also facing growing criticism from diplomats and lawmakers in both parties over his management of the department.

The latest salvo came on Tuesday, when Sens. Ben Cardin (D-Md.) and Todd Young (R-Ind.) sent Tillerson’s deputy, John Sullivan, a sharp letter expressing “significant concerns” over Tillerson’s plans to redesign the State Department and urging Tillerson and his deputy to lift the department’s hiring freeze.

The letter, obtained by Foreign Policy, underscores Congress’s growing disenchantment with Tillerson’s grand plans to overhaul the State Department, a plan that has been slammed by current and former diplomats and remains shrouded in secrecy, even to the lawmakers who oversee the department.

“The administration must enhance its transparency with Congress regarding Department of State and USAID reform and reorganization efforts,” wrote Cardin and Young, both members of the Senate Foreign Relations Committee. “The few briefings that have occurred have provided little additional insight into the ‘redesign’ efforts and raised more questions than they have answered.”

“Reforms conducted without Congress, or in opposition to the will of Congress, will be small-scale, temporary, or both,” they wrote.

Tillerson has faced a growing chorus of backlash over his efficiency drive, which encompasses an agency-wide redesign, a hiring freeze, and steep budget cuts. Critics say he is driving out the department’s top ranks of diplomats and starving the next generation of talent by curbing new hires and promotions. It comes while Tillerson tries to manage President Donald Trump’s shoot-from-the-hip approach to diplomacy, which has sometimes undermined his own efforts, and grapple with unresolved diplomatic struggles around the world, from the Gulf crisis to North Korea.

Tillerson, in Belgium on Tuesday at the start of a four-day tour of Europe, addressed these concerns in remarks at the U.S. embassy in Brussels.

“While we don’t have any wins on the board yet, I can tell you we are much better positioned to advance America’s interest around the world than we were 10 months ago, and it’s all attributable to the great men and women of this department,” he said.

He vowed to hold town halls for State Department employees before the end of the year on the redesign in order to notch some “quick wins” in reform, including a much-needed modernization of the department’s unwieldy IT systems.

Cardin and Young also took aim at Tillerson’s widely-derided justification for slashing the ranks of U.S. diplomats and shrinking their budgets. Speaking at an event in Washington on Nov. 28, Tillerson said that the administration is banking on resolving many outstanding diplomatic conflicts around the world, which means there will be less need in the future for State Department resources.

“Even if the many conflicts we confront today will subside or conclude soon—which we do not believe—it is important to remember the essential role of our diplomats before, during, and after conflicts,” the senators wrote.